Usually all children who apply for a place at a school will be admitted providing there is a space available.
If the school is oversubscribed then the school allocates places depending on their oversubscription criteria. It is often the case for faith schools that part of their criteria includes reference to faith and parents often have to demonstrate their commitment to the faith. Schools can look to the religious body representing them for assistance in this respect.
A recent case highlighting faith schools oversubscription criteria is the case of R (Governing Body of the London Oratory School v The Schools Adjudicator  EWHC 1012 (Admin)
In this case The London Oratory School is a voluntary aided Catholic academy. One of the issues discussed in the case was that the school departed from the guidance issued by the Diocese in relation to the faith element of the oversubscription criteria. The Diocese required frequency of attendance at mass to be the only test to determine whether a person was a practising Catholic, the school used various other tests.
In April/May 2013 the British Humanist Association complained to The Office of the Schools Adjudicator (OSA) about the school’s admissions process and amongst other complaints they complained the school failed to have regard to the Diocesan guidance. The OSA investigated and found that the admissions procedure was not in compliance with the Admissions Code. The school challenged this finding on the basis of judicial review and the case went to the High Court.
On the particular point regarding whether a school has to follow Diocesan guidance on the faith based part of oversubscription criteria the High Court held that:
In this case the High Court ruled that the OSA had been too stringent and should not have required the school to have a compelling reason to depart from the guidance. However, the school’s approach was also criticised and the High Court states that the over-subscription criteria was only found to be legitimate if the school could demonstrate that its unusually strong Catholic ethos taken with its pan-London missions was a clear and proper reason for departing from the guidance. This precise point was left open by the Court; however this case provides a useful insight into how much attention schools should pay to religious bodies’ guidance on faith based oversubscription criteria.